Liquid-liquid extraction, also referred to as solvent extraction or partitioning, is a method used in mineral processing to separate or extract compounds from one liquid phase into another liquid phase. This is accomplished by manipulating the relative solubilities of the compounds to be isolated in two or more liquids having differing characteristics, as is the case with an aqueous phase (such as water), and an organic solvent phase (such as an oil or immiscible organic solvent).
The term ‘solvent extraction’ can also refer to the separation of a substance from a mixture by preferentially dissolving that substance in a suitable solvent. In such a case, a soluble compound may be separated from an insoluble compound or a complex matrix.
Although the term ‘partitioning’ is sometimes used to refer to the underlying chemical and physical processes involved in liquid-liquid extraction, these terms as used herein may be considered synonymous.
In the field of solvent extraction, several hydrometallurgical systems have in the past been developed which incorporate systems for manipulating liquid flow streams and phases, and for separating, splitting, or isolating liquids or phases of liquids, and the desired compounds extracted by such systems. Typically, a mixture of an extractant in a diluent is used to extract a desired compound from one phase to another. In solvent extraction techniques this mixture is often referred to as the “organic” phase and entrained organics need to be removed or recovered from the eventual aqueous streams.
Many extraction processes make use of so-called mixer-settlers. Mixer-settlers are a type of mineral process equipment used in solvent extraction processes and consist of a first stage that mixes the phases together in an agitated tank (referred to as a mixer) followed by a quiescent settling stage, usually in the form of a gravity settling basin (settler) that allows the phases to separate by gravity. It is difficult to manage the flow of liquid in settlers to achieve sufficient separation of the various liquid phases and to minimise the settler area that is required to effect separation of the phases. The flow of liquids needs to be as laminar as possible, as flow interruptions or turbulence can lead to co-mixing of phases and resultant losses in extraction efficiency. Poor phase disengagement as a direct impact of turbulent flow also leads to, significantly higher solvent losses resulting in higher operating costs and significantly higher aqueous carryover resulting in final product impurity control issues. The settling stage allows the phases to separate, but achieving high flow rates can disturb the flow and hamper the process of separation, making it inefficient. Such systems are analysed using so-called CFD (computational fluid dynamics) modelling. Even feed distribution is important in achieving maximum possible value from the installed settler area.
In conventional mixer-settlers the intention is to distribute flow to both sides of the settler, first turning the flow through 90° and then back again through 90°. Further, reverse flow and side feed mixer-settlers distribute flow across the full width of the settler and turn the flow through 180° or 90°, respectively. However, such conventional distribution systems create recirculating zones and dead zones within the settler, in which fluid does not flow or flows in an unintended direction, often the reverse direction.
In conventional systems of which the Applicant is aware, feed distribution baffles are positioned in the feed streams in the settler area in an attempt to effect an even flow, but these systems do not produce a suitably even distribution of fluids at higher flow throughputs. A common problem with such baffles is that they are highly susceptible to scaling, leading to significant down-time and process interruption for cleaning or replacement of baffle elements. Other systems of which the Applicant is aware include fixed obstacles or deflector plates placed within the settler area, but these systems have significant drawbacks in terms of settler kinetics and flow dynamics. So-called split launders and variable-split launders have also been used in certain systems, but these are also of little value in ensuring smooth fluid flow and dispersion distribution.
The present invention has as one object thereof to overcome substantially the abovementioned problems of the prior art or to at least provide a useful alternative thereto.
The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement nor admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.
Throughout the specification and claims, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.